Abstract
An automatic test method to obtain the growth behavior of delamination fatigue cracks near the threshold region was established for unidirectional CFRP laminates by using computer-controlled fatigue testing systems. Loading apparatus for the double cantilever beam specimen and computer software were newly developed in order to make a precise measurement of the crack length by the compliance method. The resolution of the crack length increment reached to 1μm and was high enough to determine the growth threshold. The load shedding was performed in a continuous manner. A normalized gradient of the energy release rate was kept constant between -0.3 and -3mm-1 in CF/Epoxy laminates. The relation between the crack propagation rate and the fracture mechanics parameters was independent of the load-shedding condition from the threshold region to the high rate region. On the other hand, normalized gradient of the energy release rate should be kept grater than -0.9mm-1 in CF/PEEK laminates in order to get identical threshold values. This test method was applied to the CF/Epoxy and CF/PEEK laminates. Test results gave sufficient data near the threshold region for design applications.
